My daughter has an iPhone 5s and is having some problems with random text messages not being delivered to friends with Samsung devices. She does not send messages to them as iMessages, but SMS, so I know that's not the problem. Texting seems to be fine outside our home on the normal Verizon networks, but at home we have to use a network extender because we live in a rural community with no Verizon service. Many of her messages have gone through just fine in the past, but more and more frequently certain friends just do not receive her texts. Sometimes their texts will not come through to her, either. There seems to be no pattern to this, just completely random failures. She never gets a "not delivered" message to warn her, just friends who are offended that she is not answering them. This seems to happen when texts are sent to and from non-Apple devices. Is there a setting we can adjust on her phone or something related to the network extender that we can adjust?
There is something you can adjust in the phone that should help. Use iMessage and in message settings make sure 'send as SMS' is turned on.